1. INTRODUCTION

This review appears nearly a decade after the Tucson conference "Seyfert
Galaxies and Related Objects" held in February, 1968. At that time,
realization of the
similarities between the QSOs and the nuclei of Seyfert galaxies led 56
astrophysicists
to contribute their ideas regarding Seyfert galaxies [Astronomical
Journal, Vol. 73
(1968),
pp. 836-943]. A number of questions were posed regarding the energetic
events in Seyfert galaxies. Most of these remain unanswered. Progress
has been made in increasing the sample of Seyfert galaxies, in
describing the physical
conditions in their nuclei, and in delineating the relation between
Seyferts and QSOs.
Two basic aspects of the Seyfert galaxy problem have been emphasized. One is
the nature of redshifts and whether a study of Seyfert galaxies leads to
proof that QSO redshifts, by analogy, are cosmological. The other is the
search for an explanation of the extraordinary energy source that powers
the nuclei of Seyfert
galaxies. There are many reasons to believe that the same mechanism of
energy generation operates within most Seyfert nuclei as in the QSOs. To a
certain extent,
this has been a frustrating research problem because of the lack of
progress in
deciphering the fundamental mechanisms involved. On the other hand, far more
observational research is being directed toward Seyfert galaxies than
ever before, so this deserves to be chronicled.

This review makes no attempt to incorporate the numerous observations and
theories concerning the general problem of galactic nuclei, even though
this is certainly relevant to Seyfert galaxies. Some judgment had to be
used as to the point
when the review was sufficiently comprehensive to encourage new
researchers to consider the problems but not so encyclopedic as to be
intimidating. An earlier
review in this series provided a comprehensive discussion of galactic
nuclei including data on Seyferts known at the time
(Burbidge 1970).
Somewhat more recent
summaries of observational work on Seyfert galaxies are given by
Osterbrock (1971),
Sargent (1971), and
Ulrich (1974).
A comprehensive theoretical review is that of
Saslaw (1974).
Listings of Seyfert galaxies and notes on the morphology and spectra
of individual objects are given by
Khachikian & Weedman (1974)
and Adams (1977).
The relation between Seyferts and other emission-line galaxies has
recently been discussed by
van den Bergh (1975a)
and Weedman (1977).
Osterbrock (1977a)
compares the properties of Seyfert and radio galaxies. The role of
Seyfert galaxies in the redshift controversy is discussed by
Weedman (1976a).
The present review emphasizes the description of the Seyfert galaxy
phenomenon as synthesized from
recent observational studies; most of this summary is based on work done
since 1970. Unfortunately, we have so far been unable to explain much of
what we see.